X-ray tube



u y 1954 R. HOSEMANN 2,683,223

X-RAY TUBE Filed July 24, 1952 Fig. 1

Ill

Inventor";- ROLF HOSEMANN Ma i? Patented July 6, 1954 UNITED STATES ATENT OFFICE X-RAY TUBE Application July 24, 1952, Serial No. 300,587

24 Claims. 1

The present invention relates to X-ray tubes, and more particularly to X-ray tubes having a device for adjusting'the sharpness of the. image combined. with cptimal'brightness thereof. Tubes of the kind described are particularly useful for examining materials by microscopy or macroscopy' or for diagnostic purposes.

It is an object of the present invention to provide an X-ray tube employing novel means for adjusting the sharpness of the image at optimal brightness thereof.

It is another object of the present invention to provide an X-ray tube having a cathode and an anode and electrostatic means for concentrating the electron beam emitted by the cathode so as to produce a sharp image of the same on the anode.

It is a further object of the present invention to provide an analogue of a doublelen's between the cathode and the anode of the. X-ray tube.

It is still another object of the present invention to provide an analogue of an immersion-lens between the cathode and the anode of the X-ray tube, said immersion lens-focusing an electron beam emitted by the cathode at a spot near or on the anode.

It is still a further object of the presentinvention to render the double lens inserted between the cathode and the anode adjustable so as to change the position of the focal spot sothat the image of the cathode is produced beforacoinciding with, or behind the anode.

An X-ray tube according to the present invention comprises in combination, an evacuated envelope having at least a part permeable to X-ray radiation, a cathode arranged in the envelope, an anode arranged in the envelope spaced apart by a distance from the cathode, an intermediate electrode arranged in the envelope between the cathode and the anode spaced apart from the latter by a distance being a minor fraction of the distance between the cathodeand the anode, the intermediate electrode having an aperture allowing the electrons emitted by the cathode to pass, means for producing a predetermined potential difference between the anode and the cathode, and means for producing between the intermediate'electrod-e.and the cathode a potential difference being a minor fraction of the predetermined potential difference between the anode and the cathode, the potential difference between the intermediate electrode and the cathodebeing chosen so thatthe intermediate elec trode-acts' as an electrostatic double lens on the beam of electrons emitted by the cathode and produces a sharp image of the cathode on the anode.

Preferably means are provided for adjusting the ratio of the potential diiferences within limits of about 20% so as to increase the width of theimage of the cathode on the anode over the Width of the sharp image thereof.

A preferred X-ray tube according to the present invention comprises, in combination, evacuated elongated envelope having two end portions and at least'a part permeable to X-ray radiation, a cathode arranged in the envelope near one of the two end portions thereof, an anode arranged in the envelope near the other of the two end portions thereof so as to bespaced apart by a distance from the cathode, an elongated hollow intermediate'electrode arranged in the envelope'between the cathode and the anode, the elongated hollow intermediate electrode having end portions, one of the end portions being nearer to the anode than-to the cathode, a partition arranged in the one end portion of the elongated hollow intermediate electrode and being spaced apart from the anode by a distance being a minor fraction of the distance between the cathode and the anode, the partition having an aperture allowing the electrons emitted by the cathode to pass, means for producing a predetermined potential diflerenc-e between the anode and the cathode, and means for producing between the elongated intermediate electrode and the cathode a potential difference being a minor fraction of the predetermined potential difference between the anode and the cathode, the potential difference between the elongated intermediate electrode and the cathode being chosen so that the end-portions of the intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by the cathode and produce a sharp-image of the cathode on the anode.

Preferably the cathode is formed as a straight helix emitting electrons and the slot is preferably arranged parallel to the straight helix.

Preferably the elongated, for instance cylindrical hollowintermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by the cathode, the end portions of the elongated hollow intermediate electrode being arranged, respectively, nearer to the cathode and the anode forming, respectively, first and'second electrostatic simple lenses, the first electrostatic simple lens being arrangednear the cathodeacting as animmersionlens and serving asa preaccelerator and pre' concentrator for the electrons preventing largely the formation of a space charge in front of the cathode and concentrating the beam of electrons emitted by the cathode so that the same passes through the aperture of the partition arranged in the one end portion of the elongated hollow intermediate electrode, the second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of the cathode in the envelope, the Virtual image being reduced to i -V2 of the natural size of the cathode.

In a preferred embodiment of the present invention the other end portion of the cylindrical hollow electrode forms an opening having substantially the same diameter as the elongated cylindrical hollow intermediate electrode and has a distance from the cathode being to of the diameter of the cylindrical electrode.

In a preferred embodiment of the present invention the end portion of said hollow intermediate electrode being nearer to the anode serves as a member protecting against the X-ray radiation and has at least one window for allowing the X-ray radiation to pass.

In a preferred embodiment of the present invention the potential difference between the cathode and the elongated cylindrical hollow intermediate electrode and the geometrical dimensions of the second electrostatic lens are so chosen as to render negligible the lens error due to chromatic aberration transversely to the slot and to render the lens error in longitudinal direction of the slot so large that the electrons emitted by the individual turns of the straight helix overlap one another on the anode. In this manner it is accomplished that on the anode a focal linear spot having a variable width but a practically constant length is formed, the brightness of which is nearly uniform over the entire length thereof.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a longitudinal section of an X-ray tube according to the present invention together with a wiring diagram of the electrical accessories thereto;

Figs. 2 and 3 are two embodiments of a detail of the tube shown in Fig. 1 in an elevation; and

Fig. 4 is a perspective view of a tube similar to that shown in Fig. 1.

Referring now to the drawings and first to Fig. 1, the X-ray tube has an envelope i being here shown as consisting of glass. However, the material of this envelope may be impermeable to X-ray radiation except for one or more parts thereof arranged opposite to the windows l2 more fully to be described hereinafter. The X-ray tube has an elongated cylindrical shape and is provided with two end portions I and I" one of which carries a foot 1 for the electrode leads to be described more fully hereinafter. The other end portion l" carries a metallic member 3i to which a metallic cylindrical member 32 is connected having an end face 3 forming the anode of the tube. It is to be understood that the member Si is sealed to the end portion I" in an air-tight manner so that the envelope i can be entirely or partially evacuated. 0n the outside of the member 31 is provided a member 33 to which the grounded lead I5 of the voltage supply to be described hereinafter is connected. The cathode 2 is preferably shaped as a straight helix and emits the electrons. The heating current is supplied to the cathode 2 by the current leads 2' and 2", the latter being connected to a lead 16 for connecting the same to the negative high voltage end of a potentiometer ll. The high voltage is preferably equal to kilovolts.

The cathode is surrounded by a cylindrical Wehnelt electrode 4 having in the end face 4' thereof an opening 4" for accommodating the cathode 2. The cylindrical Wehnelt electrode is at about the same potential as the cathode 2 and supported by supports 5 and 8 sealed to the foot 1 of the envelope l. The support 6 is conductive and connected by a lead 24 to the high voltage end of the potentiometer H.

An elongated, preferably cylindrical, hollow intermediate electrode 8 is arranged between the cathode 2 and the anode 3. The intermediate electrode 8 has two end portions 25 and 26, the end portion 26 being arranged near the anode 3 and surrounding the same whereas the end portion 25 is arranged near the cathode 2 and provided with a circular edge 18 having substantially the same diameter as the elongated cylindrical hollow intermediate electrode 8. The opening defined by the edge I8 is preferably arranged at a distance from the cathode 2 which is equal to A; to V2 of the diameter of the elongated cylindrical hollow intermediate electrode 8. The other end portion 26 is provided with a partition 21 forming at the middle part thereof a cup-shaped member 9 provided with an aperture H! which is preferably shaped as a straight slot (Fig, 2) arranged parallel to the cathode helix 2 as shown in Fig. 4:. However, the aperture can be shaped as a circle H as shown in Fig. 3. The end portion 26 serves on the whole as a member protecting against the X-ray radiation emitted by the anode 3 and is provided with one or more windows I2 through which the X-ray radiation can escape. The intermediate electrode 8 is supported by supporting members l3 and I4 sealed to the foot I. The supporting member 16 is conductive and connected to a lead 28 connected to an adjustable tap 29 connected to the potentiometer l'l. It will be noted that the anode 3 has ground potential whereas the oathode 2 has a negative high potential with respect to the anode which preferably amounts to 50 kilovolts. The intermediate electrode 8 is at a negative high potential with respect to the anode 3 but at a positive potential with respect to the cathode 2, the potential being variable by means of the adjustable tap 29.

The operation of this device is as follows:

The intermediate electrode 3 acts as an electrostatic double lens on the beam of electrons emitted by the cathode 2. This double lens system has a double task to perform. On the one hand as many as possible of the electrons emitted by the glow cathode 2 have to be picked up, on the other hand these electrons are to be concentrated on an as small as possible surface of the anode 3, the so-called focal spot. The size of the focal spot should be variable according to the problem at hand in a simple manner. For many practical cases a minimum width of the strip-shaped focal spot of about 0.1 millimeter is sufficient. Considering that the glow cathode 2 has a dimension of approximately 1 to 2 millimeters normally to the axis of the tube, it will be seen that the lens system formed by the intermediate electrode 8 must reduce the image of the cathode t0 se /zo 0f the natural size. However, it is not essential that the lens system has no errors. On the contrary errors are desirable in certain respects because a low cathode emits electrons of different intensities at different turns of the glow cathode so that the virtual image produced by the lens system has different brightnesses at diiierent spots. These lens errors must, however, not be so large that the width of the focal spot is appreciably varied by the errors.

Such reductions not exceeding can be obtained in a particularly suitable manner by an intermediate electrode 8 kept at a suitable potential and having a partition 21 forming a cup 9. If VK is the potential of the cathode, VG that of the intermediate electrode 8, and VA=O that of the anode, and if a is the distance from the cathode 2 to the aperture and b that from the aperture l 0 to the anode 3, the following relation is found to hold at the required reduction The focal length of the simple lens formed by the end portion 26 and particularly the cupshaped member 9 is given by the following relation-- VGVK VA VG In order to obtain the virtual image of the glow cathode 2 on the surface of the anode 3, must be made approximately equal to b, that is Thus it is seen that the main portion of the voltage drop of such an X-ray tube is between the cup-shaped portion 9 of the intermediate electrode 8 and the anode 3 while the remaining space of the tube is almost free of voltage gradients. Thus, the distance b between the aperture 19 forming the boundary of the cup 9 and the anode 3 must not pass below a certain safety limit which amounts at 50 kilovolts to about 2 millimeters. The distance a between the aperture l9 forming the boundary of the cup-shaped member 9 and the cathode 2 has to be chosen according to Equation 1 and amounts to between 40 and 80 millimeters according to the voltage of the tube.

In order to prevent the formation of a stronger space charge operating as a field screen near the cathode 2 the lens system 8 is cylindrically shaped and provided with an end portion extending into the Wehnelt cylinder 4. By this it is accomplished that apart from a sufiicient pre-acceleration of the electrons in the space between the cathode 2 and the plane passing through the edge :8 of the lens 8 a slight pre-concentration of the electron beam in the space between this plane and the cup 9 is obtained so that all electrons pass through the relatively narrow aperture of the cup 9. This is accomplished by extending the len system 8 cylindrically into the Wehnelt cylinder 4 so that the edge l8 of the end portion 25 has a distance from the cathode 2 and the end face 4 which amounts to A5 to of the diameter of the cylindrical electrode 8.

It shouldbe noted that the high voltages applied to the X-ray tube and the X-ray and electron stray radiation on the anode 3 require a 6 particular shaping of the intermediate electrode 8. The end portion 25 thereof with the edge l8 acts as a weak immersion lens and the cup-shaped member 9 with the slot in as a cylindrical lens, the anode surface being situated at or near the spot where the virtual image of the cathode is formed within the end portion 26. The end portion 26 screens the walls of the envelope I against stray electrons generated at the anode 3 and acts simultaneously as a protection against the X-ray radiation which can escape through one or more Windows I2 suitably arranged in the end portion 26.

As mentioned hereinabove it is desirable that the lens system has certain lens errors by which the virtual image becomes blurred without, however, increasing appreciably the outer contour of the same. Such a lens system drawing within the inner ranges of the image diffusely but giving a relatively sharp contour is obtained according to the present invention if the glow cathode 2 and the aperture ill of the partition 21 are straight so as to generate on the anode a stripshaped focal spot of variable width but practically constant length the brightness of which is practically uniform over its entire length as shown in Fig, 4.

Fig. 4 shows several trajectories of electrons emitted by the glow cathode 2. An electron escaping with the energy zero traverses the straight line trajectory l9 ending on the anode 3. An electron escaping at the same spot with Ve traverses the trajectories 20 or 2i if its escaping velocity is parallel to the direction of the slot and the trajectories 22 or 23 if its escaping velocity is transversely directed to the trajectory l9 and the direction of the slot. If a is the distance between the cathode 2 and the edge I 8 of the intermediate electrode 8 the distance of the four trajectories 20 to 23 from the trajectory IS in the plane of the edge I8 is D 2id1 (5) The trajectories 22, 23, however, meet the middle trajectory [9 on the anode 3 after passing the slot l9. For this it is necessary that the focal length of the collecting lens 9 should be about equal to the distance I):

The focal length J is given according to Wellknown laws of electron optics by which when combined with Equation 6 gives VG-0.2VA (8) Thus, according to the invention, the potential difference between the anode 3 and the lens electrode 9 is about five times as large as that between the lens electrode 9 and the glow cathode 2, the lens electrode 9 being arranged near to the anode 3.

Since during the operation of the tube the term 7 perature of the helix forming the cathode 2 is about equal to 2400 K., Vs has the magnitude of aboutl volt. The distance D of consecutive turns of the helix amounts with technically used glow wires to'about 0.7 mm. If the distance a between the cathode 2 and the edge It is takenv as about 20 mm, inequality (5) will be satisfied if, accord ing to the invention, the following relation holds:

In the numerical example just dealt with VG has thus to be smaller than 3.2 kilovolts. This is practically sufiiciently satisfied if relation (8) is satisfied.

Thus it should be understood that the lens system 8 described hereinabove acts from an optical point of view with the geometrical and electrical dimensions described hereinabove as a cylinder lens system having in the plane of the optical cylinder lens a high chromatic error but transversely thereto a negligible chromatic error so that it satisfies the demands mentioned hereinabove.

In order to avoid any damage to the sealing of the member 32 to the envelope 1 by a radiation an annular disc I3 is arranged on the member 32.

The tap 29 allows to adjust the ratio of the potential difference of the intermediate electrode 8 and the'cathode 2 and the potential difference of the anode 3 and the cathode 2 within limits of about 20% so as to increase the width of the image-of the cathode 2 on the anode 3 produced by the electrostatic double lens over the minimum width corresponding to the optimal adjustment of the ratio of the potential differences.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of X-ray tubes differing from the types described above.

While the invention has been illustrated and described as embodiedin an X-ray tube for microscopy or diagnostic purposes, it is not intended to be limited tothe details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. An X-ray tube comprising, in combination, an evacuated envelope having at least a part permeable to X-ray radiation; a cathode arranged in said envelope; an anode arranged in said envelope spaced apart by a distance from said cathode; anintermediate electrode arranged in said envelope between said cathode and said anode spaced apart from the latter by a distance being a minor fraction of the distance between said cathode and said anode, said intermediate electrode having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said intermediate electrode and said cathode apotential differencebeing a minor fraction of the predetermined potential difference between said anode and saidcathode, said potential difference between said intermediate electrode and said cathode; being chosen so that said intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said cathode and produces an image of said cathode on said anode.

2. An X-ray tube comprising, in combination, an evacuated envelope having at least a part permeable to X-ray radiation; a cathode arranged in said envelope; an anode arranged in said envelope spaced apart by a distance from said cathode; an intermediate electrode arranged in said envelope between said cathode and. said anode spaced'apart from the latter by a distance being a minor fraction of the distance between said cathode and said anode, said intermediate electrode having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; means for producing between said intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said intermediate electrode and said cathode being chosen so that said intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said cathode and produces an image of said cathode on said anode; and means for adjusting the ratio of said potential differences within limits of about 20% so as to increase the width of the image of said cathode on said anode over the width of the sharp imag thereof.

3. An X-ray tube comprising, in combination, an evacuatd envelope having at least a part permeable to X-ray radiation; a cathode arranged in said envelope; an anode arranged in said envelope spaced apart by a distance from said cathode; an elongated intermediate electrode arranged in said envelope between said cathode and said anode, said elongated intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode, said one end portion being arranged spaced apart from the said anode by a distance being a minor fraction of the distance between said cathode and said anode, said one end portion of said elongated intermediate electrode having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difierence between said anode and said cathode, said potential difierence between said elongated intermediate electrode and said cathode being chosen so that said end portions of said inter mediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode.

4. An X-ray tube comprising, in combination, an evacuated envelope having at least a part permeable to X-ray radiation; a cathode arranged in said envelope; an anode arranged in said envelope spaced apart by a distance from said cathode; an elongated intermediate elec trode arranged in said envelope between said producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as an electrostatic .double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode; and means for adjusting the ratio of said potential differences within limits of about 20% so as to increase the width of the image of said cathode on said anode over the width of the sharp image thereof.

5. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode ar ranged in said envelope between said cathode and said anode,said elongated hollow intermediate electrode having end portions, one or said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential diiierence being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as anelectrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp'image of said cathode on said anode.

6. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X- 'ray radiation; a cathode arranged in said envelope near one of said two end portions thereof; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode; and means for adjusting the ratio of said potential difierence within limits of about 20% so as to increase the width of the image of said cathode on said anode over the width of the sharp image thereof.

7. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difierence between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difierence being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode. I

8. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X- ray radiation; a cathode arranged in said envelope near one of said twoend portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential diiTerence between said anode and said cathode, said potential diiierence between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode.

9. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions bein nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difierence between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fracl said end portions of said intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode; and means for adjusting the ratio of said potential differences within limits of about 20% so as to increase the width of the image of said cathode on said anode over the width of the sharp image thereof.

10. An X-ray tube comprising, in combination, an evacuated elongated envelope havin two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof; said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowin the electrons emitted by said helix to pass; means for producing a predetermined potential difierence between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difierence being a minor fraction of the predetermined potential diiference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions or" said intermediate electrode act as an electrostatic double lens on the beam of electrons emitted by said cathode and produce a sharp image of said cathode on said anode; and means for adjusting the ratio of said potential difierences within limits of about 20% so as to increase the width of the image of said cathode on said anode over the width of the sharp image thereof.

11. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emittin electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential diiference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said end portions of said intermediate electrode act as an electrostatic immersion lens on the beam of electrons emitted by said cathode and produce a virtual image of said helix in said envelope, said virtual image being reduced to of the size of said helix.

12. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one pass end portion of'said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by saidcathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said cathode acting as an immersion lens and serving as a rare-accelerator and preconcentrator for the electrons preventing largely the formation of a space charge in front of said cathode and concentrating the beam of electrons emitted by said cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said cathode in said envelope, said virtual image being reduced to Vie-V20 of the natural size of said cathode.

'13. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition havingan aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difierence between said'anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated in termediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical static simple lens being arranged near said'helical cathode acting as an immersion lens and serving as a pre-accelerator and pre-concentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to /10 /20 of the natural size of said helical cathode.

14. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end ortions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated l1ollow'in+ termediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and. said oath.- ode a potential difference being a minor fraction of the predetermined potential dilference between said anode and said cathode, said potential difference between said elongatedintermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical oathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said helical cathode acting as an immersion lens and serving as a pre-accelerator and pie-concentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and. concentrating the beam of electrons emitted by said helical cathode so that the samev passes through said aperture of said partition arranged in one portion of elongated hollow intermediate electrode, said second electrostatic simple lens having a focallength of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to r yoof the natural size of said hello i cathode. 1 v

, 15. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a permeable to X-ray radiation; a cathode arranged in said envelope near one ofsaid two end portions thereof; an anode arranged-in said envelope near the'oth'er of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difi'erence between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said cathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said cathode acting as an immerison lens and serving as a pre-accelerator and pre-concentrator for the electrons preventing largely the formation of a space charge in front of said cathode and concentrating the beam of electrons emitted by said cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said cathode in said envelope, said virtual image being reduced to of the natural size of said cathode; and means for adjusting the ratio of said potential differences within limits of about 20% so as to increase the width of the image of said cathode on said anode produced by said electrostatic double lens over the width of the sharp image thereof.

16. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions therei of, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elon-- gated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difierence between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difierence between said elongated intermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said helical cathode acting as an immersion lens and serving as a pre-accelerator and preconcentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to rte- & of the natural size of said helical cathode; and means for adjusting the ratio of said po-- tential differences within limits of about 20% so as to increase the width of the image of said helical cathode on said anode produced by said electrostatic double lens over the width of the sharp image thereof.

1'7. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said helical cathode acting as an immersion 17 lens and serving as a pre-accelerator andpre concentrator for the electrons preventing largely the formation-of a space charge in front of said helical cathode and concentra'ting'the beam of electrons emitted by said helical cathode so thatthe same passes through said aperture of said partition arranged in said one end'p'o'rtionof said elongated hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as 'to'pr'oduce' a virtual image of said helical cathode in said envelope,- said virtual image being reduced to' re /2 of the natural size of said helicalc'athode; andm'eans for adjusting the ratio of said potential differences within limits of about 20% so as to increase the widthof the image of said near one of said two end portions thereof; an

anode arranged in said envelope near the other of's'aid'two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated cylindrical hollow intermediate electrode" arranged in said envelope between s'a'idcatho'de and said anode, said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than'to said cathode, the other of saidend portionsformin'g' an opening having substantially the same diameter as said' elongated cylindrical hollow intermediate electrode and having a distance' from said cathode being /5 to /2 of the" diameter of said elongated cylindrical hollow intermediate electrode; a partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential difference between said anode and said'cathode; and means for producing between said elongated intermediate electrodeand said cathode-a potential difference being'a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated cylindrical hollow intermediate electrode acts as an electrostatic'double lens on the beam of electrons'emitted by said cathode, said end portions of said elongated cylindrical hollow intermediate electrode being arranged, respectively, nearer to said cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens formed by said opening and being arranged near said cathode acting as an immersion lens and serving as a pro-accelerator and pre' concentrator for the electrons preventing largely the formation of a space charge in front of said cathode and-concentrating the beam of electrons emitted by said cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode; said second electrostatic simple lens hav-' ing a focal length of a few millimeters so as't'o produce a virtual image of said cathode" in said 18 envelope, said virtual image being reduced to f d/ of the natural size of said cathode.

I9. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at'leastia, part permeable to'x ray radiation; a cathode arranged in said envelope ne'ar'one of said two end portions thereof, said cathode beingformed a's a strai hthelix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as tobe spaced apartby a distance from'said cathode; an elongated h'ollow' cylindrical intermediate electrode" arranged" in said envelope between said cathode and said anode, said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being. nearert'c s'ai'danode than to said'cathode, the" other of "said end portions forming an opening having substantially the same diameter as said elongated cylindrical hollow intermediate electrode and having a distance from s'aid'helical cathode being /5 to /2 of the diameter of said elongatedcylindrical hollow intermediate electrode; a partition arranged in said one endportion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said'anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass;

means for producing; a predetermined potential difference between said anod'e'andsa-id cathode; and" means for producing between said elongated intermediate electrodeand said cathode a potential' difference beinga minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and'said cathode being chosen so that said elongatedcylindrical hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated cylindrical hollow intermediate electrode being arranged, respectively, nearento said' helical cathode and said anode forming, respectively, first and second electrostatic' simple lenses, said first electrostatic simple lens formed by said opening and being arranged near'said helical cathode acting as an immersion lens and serving as a pre-accelerator and pro-concentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that thesame passes through said aperture of said partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to T o" /2O of the natural siz of said helical cathode.

' 20. An X-ray tube comprising, in combination, an evacuated elongated-envelopehaving two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one ofsaid two end portions'thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said-two -end portions thereof" so as to be spaced apart'by a distance from said cathode; an elongated hollow cylindrical intermediate electrode arranged in said envelope between said cathode and said anode;

acs aaae i ii said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode, the other of said end portions forming an opening having substantially the same diameter as said elongated cylindrical hollow intermediate electrode and having a distance from said helical cathode being to of the diameter of said elongated cylindrical hollow intermediate electrode; a partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between saild anode and said cathode; and means for produring between said elongated intermediate elec trode and said cathode a potential difference being a minor fraction of the predetermined potential diiference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated cylindrical hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated cylindrical hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens formed by said opening and being arranged near said helical cathode acting as an immersion lens and serving as a pre-accelerator and pro-concentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to of the natural size of said helical cathode.

21. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions there: of; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow intermediate electrode arranged in said envelope between said cathode and said anode, said elongated hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode; a partition arranged in said one end portion of said elongated hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potential dif ference between said anode and said cathode; and means for producing between said elon'gatedifintermediate electrode and said cathode a poten 2G tial diflference being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said cathode, said end portions of said elongated hollow intermediate electrode being arranged, respectively, nearer to said cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens being arranged near said cathode acting as an immersion lens and serving as a pre-accelerator and pre-concentrator for the electrons preventing largely the formation of a space charge in front of said cathode and concentrating the beam of electrons emitted by said cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated hollow intermediate electrode, said second end portion of said elongated hollow intermediate electrode serving as a member protecting against the X-ray radiation and having at least one window for allowing the X-ray radaition to pass, said second electrostatic simple lens having a focal length of a few millimeters so as to produce a virtual image of said cathode in said envelope, said virtual image being reduced to 1A0-J/20 of the natural size of said cathode.

22. An X-ray tube comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated cylindrical hollow intermediate electrode arranged in said envelope betweensaid cathode and said anode, said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode, the otherof said end portions forming an opening having substantially the same diameter as said elongated cylindrical hollow intermediate electrode and having a distance from said cathode being /5 to of the diameter of said elongated cylindrical hollow intermediate electrode; a partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said anode by a dis tance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture allowing the electrons emitted by said cathode to pass; means for producing a predetermined potentialdifference between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difierence being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated cylindrical hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by'said cathode, said end portions or" said elongated cylindrical hollow intermediate elec-- trode being arranged, respectively, nearer to said cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens formed by said 21 opening and beingjarrange'd nearsaid cathode acting as an immersionlens and serving'as a preaccelerator andjpre-concentratorfor the electronspreventing.largelythe formationiof a space charge in'front of said cathode and concentrating the beam of electrons emitted'by said cathode so thatthe same passes through said aperture of said partition arranged in said one end portion of said elongated cylindrical hollow intermediate. electrode, said second electrostatic simple lens havingafocal length: of a few millimeters seas-to produce avirtual, image of saidcathode; in, said, envelope, said virtual image being reduce to of the natural size of said cathode; and means for adjusting the ratio of said potential difierences within limits of about 20% so as to increase the width of the image of said cathode on said anode produced by said electrostatic double lens over the width of the sharp image thereof.

23. An X-ray tube, comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow cylindrical intermediate electrode arranged in said envelope between said cathode and said anode, said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode, the other of said end portions forming an opening having substantially the same diameter as said elongated cylindrical hollow intermediate electrode and having a distance from said helical cathode being /5 to /2 of the diameter of said elongated cylindrical hollow intermediate electrode; a partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between said anode and said cathode; and means for producing between said elongated intermediate electrode and said cathode a potential difierence being a minor fraction of the predetermined potential difference between said anode and said cathode, said potential difference between said elongated intermediate electrode and said cathode being chosen so that said elongated cylindrical hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated cylindrical hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens formed by said opening and being arranged near said helical cathode acting as an immersion lens and serving as a pre-accelerator and pre-concentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that the same passes through said aperture of said partition arranged 22 in said one end portion of said elongated cylin 'drical' hollow intermediate electrode, said second: electrostatic simple lens having 'a focal length of a few millimeters was to products virtual image of said helical cathode in said'envelopesaid virtual image being-reduced to /g of the natural size of said helical cathode, saidp'oterstial difierence between said cathodeand-said elongated cylindrical hollow intermediate 'electrode and the geometrical dimensions of'sa-id seeond electrostatic lens being" so chosen as to ren der negligible the lens error due to chromatic aberration transversely to said'slot and" to rendersaid lens error inlongitudinal direction of 'said slot so large that the electrons emitted by the individual turns of said straight helix overlap one another on said anode.

24. An X-ray tube, comprising, in combination, an evacuated elongated envelope having two end portions and at least a part permeable to X-ray radiation; a cathode arranged in said envelope near one of said two end portions thereof, said cathode being formed as a straight helix emitting electrons; an anode arranged in said envelope near the other of said two end portions thereof so as to be spaced apart by a distance from said cathode; an elongated hollow cylindrical intermediate electrode arranged in said envelope between said cathode and said anode, said elongated cylindrical hollow intermediate electrode having end portions, one of said end portions being nearer to said anode than to said cathode, the other of said end portions forming an opening having substantially the same diameter as said elongated cylindrical hollow intermediate electrode and having a distance from said helical cathode being /5 to of the diameter of said elongated cylindrical hollow intermediate electrode; a partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode and being spaced apart from said anode by a distance being a minor fraction of the distance between said cathode and said anode, said partition having an 1 aperture formed as a slot arranged parallel to said straight helix and allowing the electrons emitted by said helix to pass; means for producing a predetermined potential difference between said anode and said cathode; means for producing between said elongated intermediate electrode and said cathode a potential difierence being a minor fraction of the predetermined potential difierence between said anode and said cathode, said potential difierence between said elongated intermediate electrode and said cathode being chosen so that said elongated cylindrical hollow intermediate electrode acts as an electrostatic double lens on the beam of electrons emitted by said helical cathode, said end portions of said elongated cylindrical hollow intermediate electrode being arranged, respectively, nearer to said helical cathode and said anode forming, respectively, first and second electrostatic simple lenses, said first electrostatic simple lens formed by said opening and being arranged near said helical cathode acting as an immersion lens and serving as a pre-accelerator and preconcentrator for the electrons preventing largely the formation of a space charge in front of said helical cathode and concentrating the beam of electrons emitted by said helical cathode so that the same passes through said aperture of said partition arranged in said one end portion of said elongated cylindrical hollow intermediate electrode, said second electrostatic simple lens hav- 23 ing a focal length of a few millimeters so as to produce a virtual image of said helical cathode in said envelope, said virtual image being reduced to 1 6 /20 of the natural size of said helical cathode, said potential difierence between said cathode and said elongated cylindrical hollow intermediate electrode and the geometrical dimensions of said second electrostatic lens being so chosen as to render negligible the lens error due to chromatic aberration transversely to said slot and to render said lens error in longitudinal direction of said slot so large that the electrons emitted by the individual turns of said straight helix overlap one another on said anode;

24 and means for'adjusting the ratio of said potential differences within limits of about 20% so as to increase the width of the image of said helical cathode on said anode produced by said electrostatic double lens over the width of the sharp image thereof.

References Cited in the file Of this patent UNITED STATES PATENTS Douma Nov. 4, 1952 

